At various points during Chapters II and III, a Druid (not the Hierophant nor the Elder druid) in Druids' grove will give you this potion if you talk to him. He mentions that you aid the Brickmakers, and this potion grants immunity to being stunned and confused which is useful when dealing with the Brickmakers. Whether or not he gives it seems to depend on progress in the various quests and time of day or some random chance, as it can be difficult to recreate the conversation from saved games.

Receiving this potion from the druids in Chapter V, during "The Green March" will unlock the formula if you did not have it already.